Document processing devices typically include one or more sets of nips used to transport media (i.e., sheets) within the device. A nip provides a force to a sheet as it passes through the nip to propel it forward through the document processing device. Depending upon the size of the sheet that is being transported, one or more nips in a set of nips might not contact the sheet as it is being transported.
FIG. 1A depicts a top view of a portion of an exemplary document processing device known in the art. As shown in FIG. 1A, the document processing device 100 includes three sets of nips 105a-b, 110a-b, and 115a-b. The first set of nips 105a-b are used to transport a sheet; the second set of nips 110a-b are used to perform sheet registration; and the third set of nips 115a-b are used to transport a sheet in a process direction. Although two nips are shown for each set of nips, additional or fewer nips can be used. In some cases, additional nips are used to account for variations in sheet size during the transport or registration processes.
As shown in FIG. 1B, each nip in a set of nips, such as 115a-b, includes a drive wheel, such as 125, and an idler wheel, such as 130. A normal force is caused at each nip by loading the idler wheel 130. Friction between the sheet and each nip 115a-b is used to produce a normal force that propels the sheet in a process direction. Typically, each idler wheel 130 is mounted independently from the other idler wheels in a set of nips.
Transferring a sheet in the process direction to consecutive sets of nips 115a-b or to another station within a document processing device 100 (e.g., to receive an image from a photoreceptor) requires each nip pair to open and close. In conventional systems, the idler wheels 130 are part of a moveable mechanism connected to an actuator that opens and closes the nip. When traveling through the nip, the trajectory of the sheet is greatly influenced by the alignment of the idler wheels. Therefore, the proper alignment of the idler wheels is important to achieve accurate and repeatable sheet motion and prevent skewing of the sheet. With the idler wheels repeatedly moving between the open and closed position, the proper alignment thereof is difficult to maintain.